Picture a thunderstorm - heavy raindrops beat the roof, lightning flashes through the windows, thunder booms, the dog whines from his hiding spot under your bed. Now picture two thousand thunderstorms.
Right now, at this very moment, there are about two thousand thunderstorms going on around the world. Even though thunderstorms are common, they are still dramatic events with intense rain, hail, wind, lightning, thunder, and even tornadoes.
Thunderstorms form when warm, moist air rises into cold air. The warm air becomes cooler, which causes moisture, called water vapor, to form small water droplets - a process called condensation. The cooled air drops lower in the atmosphere, warms and rises again. This circuit of rising and falling air is called a convection cell. If this happens a small amount, a cloud will form. If this happens with large amounts of air and moisture, a thunderstorm can form.
Thunderstorms can consist of just one convection cell, multiple convection cells, or even one extremely large and powerful convection cell. Below is a description of three types of thunderstorms, classified by their structure: single-cell, mulit-cell and supercell.
- Single-cell Thunderstorms: Thunderstorms created by just one convection cell in the atmosphere are called single-cell storms. Most of these are small, lasting only about an hour, and are also called ordinary thunderstorms. These storms often form during summer and include towering cumulonimbus clouds that can grow 12 kilometers high in the atmosphere. Rain and lightning are common. Sometimes hail falls.
- Multi-cell Thunderstorms: Some thunderstorms are made from many convection cells moving as a single unit. These are called multi-cell thunderstorms. Often the convection cells are arranged as a cluster, with each cell at a different stage of the thunderstorm cycle. Multi-cell storms along a cold or warm front, where warm air is pushed high into the atmosphere above cold air, often form a line, called a squall line. The squall line can be up to 600 miles (1000 km) long. Strong wind gusts often blow just ahead of the storm.
- Supercell Thunderstorms: Thunderstorms with deep, rotating updraft winds, called supercells, are very large and last for hours releasing huge amounts of rain and sometimes even baseball-sized hail. They include fast moving convection – air zooming upward at as much as 175 miles (280 km) per hour. Rotation in supercells sometimes forms violent tornadoes, the largest and most damaging type, because the storms are so long-lived. Several tornadoes can be produced from one supercell thunderstorm. And clouds grow up to 18 km in the atmosphere. Supercells are the least common type of thunderstorm.
Where Air Rises to Form a Thunderstorm
All thunderstorms begin with air rising into the atmosphere to form a convection cell, but the air can be lifted in different ways. Another way to classify thunderstorms is by the location where they form and the reason that air rises. The pictures below describe three different ways that lifting of air can begin: due to a mountain or hillside, within an air mass, or at a storm front.
Orographic thunderstorms are caused by air that is forced up by a mountain or hillside.
Air mass thunderstorms are the result of localized convection in an unstable air mass.
Frontal thunderstorms occur along boundaries of weather fronts (e.g. cold front).